This invention relates generally to air conditioning systems and methods for providing conditioned air to the cabin or cockpit of an aircraft. More specifically, this invention relates to an improved aircraft air conditioning system of the type designed to utilize pressurized bleed air from one or more aircraft engines, wherein the engine bleed air is supplemented by a substantial fresh air inflow throughout a broad range of low to high altitude operation.
Air conditioning and/or cabin pressurization systems for aircraft are generally known in the art wherein engine bleed air is used as a source of fresh air which can be supplied to the aircraft cabin or cockpit at a selected temperature, pressure, and relative humidity. In such systems, a portion of a compressed air supply from an engine compressor is diverted or bled off from the engine to drive a turbine which expands and cools the otherwise heated high pressure air in preparation for supply to the aircraft cabin. The expansion turbine is normally associated with one or more heat exchangers and/or water extraction devices for conditioning the air to the desired parameters for the comfort of cabin occupants. While such systems operate in a generally satisfactory manner, the reduced ambient air pressure at higher altitudes inherently requires an increased proportion of the engine air flow to be diverted to the aircraft cabin to maintain the requisite cabin air flow. This diversion of air from the engine undesirably decreases engine fuel efficiency and thus directly results in increased engine fuel consumption.
In the past, modified aircraft air conditioning systems have been proposed in an effort to reduce requirements for engine air. For example, systems have been developed wherein the expansion turbine is used to drive a compressor which draws in and compresses additional fresh air as a supplement to the engine bleed air. In these systems, the compressed fresh air is added to the engine bleed air to provide a combined pressurized flow which is expanded and cooled by driving passage through the turbine. Unfortunately, however, the decreasing pressure of ambient air at increasing altitude results in excessive compressor power consumption and/or prohibitive compressor pressure boost ratios at high altitude operation. Some systems have attempted to avoid these disadvantages by recirculating cabin air through the compressor during high altitude operation, but this approach generally does not provide the cabin with a desired level of incoming fresh air. Instead, increased fresh air has been available only by increasing the proportion of bleed flow from the engine, with accompanying fuel penalty.
There exists, therefore, a significant need for an improved aircraft cabin air conditioning system designed to provide a substantial fresh air inflow as a supplement to engine bleed air throughout a broad range of low to high altitude operation, and thereby substantially minimize system requirements for engine bleed air to correspondingly improve overall engine fuel efficiency. The present invention fulfills these needs and provides further related advantages.